The phenomenon of scour, i.e., to clear or remove by a current of fluid, is a well known problem associated with structures supported in bodies of moving water. Scouring mechanisms are present around and associated with any secured structure in the path of a moving fluid. Scour is especially important when trying to maintain the structural integrity of gravity supported structures embedded in a sediment floor beneath a body of moving water.
The scour of sediment from around structures embedded in the floor beneath a body of moving water is a difficult problem. Scour is encountered when the following criteria exist a moving fluid, a relatively fixed structure and a movable material associated with the structure. Scour is the progressive removal of the material from around or underneath the structure.
Specifically, the scouring process is effective on any material associated with an object in the path of a moving fluid. Scour is a universal phenomenon that is often seen or experienced. For example, any person who stands on a beach and allows the water to pass around and by his feet to dislodge the sand from around and under his feet is experiencing scour.
Commercially, scour is a critical problem in many offshore areas where large, heavy structures such as offshore drilling and production platforms must be supported from the floor beneath the ocean. Likewise, scour is a critical factor in designing stanchions that support bridges spanning rivers or other bodies of moving water. Scouring critically reduces the support supplied to a gravity supported structure.
It is well known to retard the scouring process by using sand bags to refill the holes caused by the removed material. Even the relocation of the structure has proven to be economically feasible to circumvent excessive scouring problems.
More recently, numerous and varied methods of preventing or retarding scouring have been utilized. A screen-mesh material has been used to cover the floor around a structure beneath the body of water. Screen-mesh material has proven helpful in the immediate vicinity of the structure, but has caused excessive scour to a larger region about the screen-mesh material. Alternate methods of preventing scour have included the implantation of more dense material about the structure. Examples of material used is concrete and asphalt. The implantation methods have resulted in even more acute scouring problems depending on the depth and width of the implant. It is even common practice in many offshore areas with large structures that exhibit significant scouring to use commercial divers. The divers move along the floor beneath the body of water using devices that blow the bottom sediment from an adjacent area into the holes around the structures caused by scour.
All of the presently known or used devices and methods for preventing, reducing or repairing scour address the effects of the scouring process. The devices and methods, some of which are discussed above, are merely engineering techniques that have been developed to deal with a common and universal problem. None of the discussed devices or methods seeks to address the cause of scouring.
Numerous and varied factors can enhance the scouring phenomenon. Of significant importance is the consistency of the material in which the structure is embedded. A sea bed consisting of non-cohesive material is extremely susceptible to scouring forces. Thus, a floor beneath a body of water that consists substantially of silty material, sand or gravel is highly susceptible to the scouring process. The scouring process is enhanced by the presence of such non-cohesive material, since scouring requires the disengagement, suspension and movement of the floor sediments.
Another critical parameter associated with the scouring process is the current velocity of the fluid. There exists a critical current velocity associated with, but not exclusive of, the geometry of the structure and the material or sediment to be transported. Thus, a critical current velocity required to initiate scour can be expressed as a function of the following parameters: the geometric shape of the structure, the size of the sediment material to be transported, the density of the sediment material to be transported and the shape of the sediment material to be transported.
Scour can adversely effect the structural stability of any object around which the phenomenon takes place. For example, a stanchion embedded in non-cohesive sediment beneath a body of moving water will experience significant scour around the stanchion. As the sediment is removed from around the stanchion, the cross-sectional area of the stanchion exposed to the force delivered by the flow of water is increased. An increase in the encountered cross-sectional area provides an increase in the total force adverse to the stability of the stanchion.
Similarly, as the sediment is removed from around the stanchion, the effective contact area of the floor sediment with the structure decreases. The effective contact area of the floor sediment with the structure is directly proportional to the force required to dislodge the structure. As the effective contact area decreases, the force required to be exerted on the structure by the water impacting thereupon is also reduced. Therefore, the stability of the structure is reduced since the structural support provided by the floor sediment in contact with the structure has been reduced.
As the scour phenomenon proceeds and floor sediment is removed, an additional consideration is the shift in the fulcrum about which the stanchion pivots. The fulcrum point for a gravity supported structure embedded in non-cohesive sediment would typically be at some location in the sediment below the sediment-water interface. A small change in the location of the fulcrum causes a change in the lever arm distances associated with the system and causes a significant change in the forces related to the securement and the dislodgement of the stanchion. A small change in the lever arm distances, dramatically changes the relative magnitudes of the forces. Thus, the force associated with the current instigating dislodgement is dramatically increased and the force associated with the floor sediment maintaining securement of the stanchion is dramatically decreased. Therefore, the amount of and the rate of scour is extremely important for the stability of any structure embedded in material beneath a body of moving water.
There is thus a need for an anti-scour apparatus which can be associated with a structure embedded in material beneath a body of moving fluid, which, at the same time, provides superior structural integrity, which is readily built into a new structure or assembled and installed on an old structure, and which is exceedingly less expensive than all prior known devices and methods.
It is, therefore, a feature of the present invention to provide a unique anti-scour apparatus and method for implementation with structures embedded in the floor beneath a body of moving water.
It is a more particular feature of the present invention to provide an anti-scour apparatus and method to manipulate and control the transport mechanisms associated with the movement of material around a structure in a fluid flow.
Another feature of the present invention is to provide an anti-scour apparatus and method for controlling the flows that cause scour.
Yet another feature of the present invention is to prevent the impingement, on the sediment material supporting a structure, of the downwash associated with the impact of moving water on the structure.
Yet still another feature of the present invention is to provide an antiscour apparatus and method that reduces the size of the separation of the fluid flow associated with a structure, thereby reducing the wake downstream of the structure.
A further feature of the present invention is to provide an antiscour apparatus and method for delaying the separation of the fluid flow which reduces the magnitude of the drag on the structure.
Still further a feature of the present invention is to provide an anti-scour apparatus and method to prevent the impact of a downwash, associated with a fluid flow and a structure therein, on the bottom sediment preventing a transfer of energy from the downwash to the sediment.
Still further a feature of the present invention is to provide an anti-scour protector and method to prevent, downstream of a structure in a moving fluid, the filling of the wake with and being the transport mechanism for bottom sediment.
Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The features and advantages of the invention may be realized by means of the combinations and steps particularly pointed out in the appended claims.